Thin films of iron sulfide, especially, iron disulfide (pyrite FeS2), are expected as a light absorbing material in film solar cells, and have been actively developed in recent years. In order to put such metal sulfide films on the market as industrial products, not only high quality but also mass productivity at a low cost is requested. Technologies satisfying such requirements have so far been developed.
Examples of a preparation method of such a metal sulfide film include film preparing methodes under vacuum such as MOCVD or sputtering, and sulfurization by the heat treatment of a metal film.
MOCVD (Metal Organic Chemical Vapor Deposition) is one of the CVD methodes in which a film is formed on a substrate by reacting a raw material at high temperatures, and is particularly a method that uses an organic metal as the raw material. The film preparation method by MOCVD is, for example, disclosed in Journal of Crystal Growth, 151, p. 325, (1995).
The film preparation method by sputtering is a method of spraying an inert gas to a material such as metal and attaching molecules thus sputtered to the surface of an object, and it is disclosed, for example, in Thin Solid Films, 246, p. 6, (1994).
These preparation methodes however involve many problems upon industrialization, for example, they need expensive vacuum systems or expensive raw materials such as TBDS, a film forming rate is low, or the number of steps for the method is large. In particular, iron sulfide exists in the various forms such as troilite FeS, pyrrhotite Fe1-xS, pyrite FeS2, and marcasite FeS2, and thus it is necessary to set strict conditions for the preparation of a good-quality film composed of only a single phase among these various forms. Especially, there is a considerable difficulty in preparation of a film of iron disulfide (pyrite FeS2) which is expected as a material for film solar cells. Owing to the existence of a sulfur defect, the resulting film partially becomes, for example, pyrrhotite Fe1-xS, meaning the preparation of a multiphase film.
As a preparation method of a metal sulfide film, a method is proposed in which a Fe2O3+Cu2O film or Fe+S film is formed, and subsequently the film is heated in a sulfur-containing atmosphere to sulfurize the metal, in Japanese Patent Application Unexamined Publication No. 8-199333/1996 A and Japanese Patent Application Unexamined Publication No. 8-218161/1996 A. This method is however complicated and needs post-treatment such as heat treatment after film formation. In addition, it is difficult to prepare a single-phase film with good quality, because strict conditions are required for completely sulfurizing the metal by heat treatment.